1. Field of the Invention
This invention relates generally to actuation of a motor vehicle park brake that produces engagement, disengagement and flat-tow operation of a park brake operated by a shift-by wire system.
2. Description of the Prior Art
Parking mechanisms or assemblies are used in automotive vehicles to allow the vehicle to enter into and maintain a “parked” condition or state. Parking assemblies typically include a park rod, which selectively actuates a parking pawl that operatively engages the vehicle's park gear, thereby maintaining the vehicle in a “parked” position or state.
The park rod is typically mechanically linked to the vehicle's shifter and moves in response to shifter movement. When the shifter is moved to the “park” position, a contact member, which is disposed on the park rod, engages the parking pawl, which pivots or rotates into a position where it is disposed between a pair of teeth on the park gear, thereby substantially preventing the park gear and the output shaft from further rotating.
The park rod includes a spring which is sometimes compressed during the engagement with the parking pawl. Particularly, the spring within the park rod is compressed only during certain shifts into park. For instance, when the park rod contact member engages the parking pawl and causes it to pivot into the park gear, the parking pawl is sometimes pivoted into and forcibly abuts a tooth of the park gear. During these types of shifts, the pivotal movement of the parking pawl is temporarily limited or stopped, thereby preventing further movement of the contact member until the park gear is rotated and causing the spring within the park rod to be compressed. When the park gear rotates, the parking pawl slides along the tooth which it abuts until it “drops into” or is forced into a space between teeth of the park gear. During other shifts into the park position, the parking pawl pivots directly into a space between gear teeth. During these types of shifts, the path of travel of the park rod is substantially unobstructed and the spring within the park rod is not compressed. The resulting inconsistency of spring compression causes the force required to shift the vehicle into park to vary from shift to shift. Because the shifter is mechanically coupled to the park rod in these prior systems, the user or driver of the vehicle undesirably “feels” or experiences inconsistent or different forces at the shifter when shifting the vehicle into park. Furthermore, when shifting a vehicle out of the park position while the vehicle is disposed on an incline, the loading of the park pawl will vary with the loading of the vehicle. This causes the park “pull out” load to vary based upon the vehicle weight and incline. This load variation is likewise undesirably experienced by an operator of the vehicle.
Another drawback with these prior mechanical systems occurs when a driver inadvertently shifts into park while the vehicle is still moving. Particularly, if the vehicle is moving above a certain speed, the parking pawl may not fully engage the park gear, but rather “ratchets” against the gear, thereby creating an unaesthetic sound and vibration and potentially damaging the parking pawl and park gear. Moreover, when residual torque remains within the output shaft during a parking event (e.g., when the vehicle is parked on an inclined or declined surface), the park gear imparts a torsional force or load on the parking pawl which may be transmitted through the park rod and may result in undesirable vibrations which can be “felt” or experienced at the vehicle's shifter.
Vehicles which utilize a “shift by wire” system, where the shifter is mechanically coupled to and actuates the park rod by use of a wire or cable, suffer from other drawbacks. By way of example and without limitation, the cable may act as a noise and/or vibration path or medium which generates undesirable noise and/or vibration that can be experienced within the passenger compartment.